Calcium homeostasis is the mechanism by which the body maintains adequate calcium levels. The process is highly regulated, and involves a complex interplay between calcium absorption, transport, storage in bones, deposition in other tissues, and excretion. PTH is a regulator of circulating calcium levels, and functions to increase the concentration of calcium in the blood by enhancing the release of calcium from bone through the process of bone resorption; increasing reabsorption of calcium from the renal tubules; and enhancing calcium absorption in the intestine by increasing the production of 1,25-(OH)2 vitamin D, the active form of vitamin D. PTH also stimulates phosphorus excretion from the kidney, and increases release from bone.
PTH secretion is regulated by the calcium sensing receptor (CaSR), a G-protein coupled receptor expressed by several cell types on the surface of parathyroid cells, which detects small fluctuations in the concentration of extracellular calcium ion (Ca2+) and responds by altering the secretion of PTH. Activation of the CaSR by Ca2+ inhibits PTH secretion within seconds to minutes through inhibition of vesicular transport, and this process may be modulated by protein kinase C (PKC) phosphorylation of the receptor. The CaSR is also expressed on osteoblasts and in the kidney, where it regulates renal Ca2+ excretion.
In addition, PTH regulates phosphorus homeostasis. PTH stimulates the parathyroid hormone receptor 1 (PTHR1) on both apical (brush border membrane) and basolateral membranes of cells in the GI tract. PTHR1 stimulation leads to an increase in urinary excretion of phosphate (Pi) as a consequence of reduction by internalization of the renal Na+/phosphate (NaPi-Ila) co-transporter on the brush border membrane.
PTH is also involved in the regulation of osteoblasts and osteoclasts in bone. PTH increases circulating Ca2+ by increasing bone resorption and renal reabsorption of calcium. PTH stimulates osteoblasts to produce RANK ligand (RANKL), which binds to the RANK receptor and activates the osteoclasts, leading to an increase in bone resorption and an increase in serum Ca2+. Osteoprotegerin (OPG) is a decoy receptor for RANKL which blocks bone resorption. Osteoporosis is caused by an imbalance between the processes of bone resorption by osteoclasts and bone formation by osteoblasts.
The human body contains approximately 1 kg of calcium, 99% of which resides in bone. Under normal conditions, circulating calcium ion (Ca2+) is tightly maintained at a level of about 9 to 10 mg/dL (i.e., 2.25-2.5 mmol/L; ˜600 mg). Approximately 1 g of elemental calcium (Ca2+) is ingested daily. Of this amount, approximately 200 mg/day is absorbed, and 800 mg/day is excreted. In addition, approximately 500 mg/day is released by bone resorption or is deposited into bone. About 10 g of Ca2+ is filtered through the kidney per day, with about 200 mg appearing in the urine, and the remainder being reabsorbed.
Hypercalcemia is an elevated calcium level in the blood. Acute hypercalcemia can result in gastrointestinal (anorexia, nausea, vomiting); renal (polyuria, polydipsia), neuro-muscular (depression, confusion, stupor, coma) and cardiac (bradycardia, first degree atrio-ventricular) symptoms. Chronic hypercalcemia is also associated with gastrointestinal (dyspepsia, constipation, pancreatitis); renal (nephrolithiasis, nephrocalcinosis), neuro-muscular (weakness) and cardiac (hypertension block, digitalis sensitivity) symptoms. Abnormal heart rhythms can result, and EKG findings of a short QT interval and a widened T wave suggest hypercalcemia. Hypercalcemia may be asymptomatic, with symptoms more commonly occurring at high calcium levels (12.0 mg/dL or 3 mmol/l). Severe hypercalcemia (above 15-16 mg/dL or 3.75-4 mmol/l) is considered a medical emergency: at these levels, coma and cardiac arrest can result.
Hypercalcemia is frequently caused by hyperparathyroidism, leading to excess bone resorption and elevated levels of serum calcium. In primary sporadic hyperparathyroidism, PTH is overproduced by a single parathyroid adenoma; less commonly, multiple adenomas or diffuse parathyroid gland hyperplasia may be causative. Increased PTH secretion leads to a net increase in bone resorption, with release of Ca2+ and phosphate (Pi). PTH also enhances renal reabsorption of Ca2+ and inhibits reabsorption of phosphate (Pi), resulting in a net increase in serum calcium and a decrease in phosphate.
Secondary hyperparathyroidism occurs when a decrease in circulating levels of Ca2+ level stimulates PTH secretion. One cause of secondary hyperparathyroidism is chronic renal insufficiency (also referred to as chronic kidney disease or CKD), such as that in renal polycystic disease or chronic pyelonephritis, or chronic renal failure, such as that in hemodialysis patients (also referred to as end stage renal disease or ESRD). Excess PTH may be produced in response to hypocalcemia resulting from low calcium intake, GI disorders, renal insufficiency, vitamin D deficiency, and renal hypercalciuria. Tertiary hyperparathyroidism may occur after a long period of secondary hyperparathyroidism and hypercalcemia.
Malignancy is a common cause of non-PTH mediated hypercalcemia. Hypercalcemia of malignancy, is an uncommon but severe complication of cancer, affecting between 10% and 20% of cancer patients, and may occur with both solid tumors and leukemia. The condition has an abrupt onset and has a very poor prognosis, with a median survival of only six weeks. Growth factors (GF) regulate the production of parathyroid hormone-related protein (PTHrP) in tumor cells. Tumor cells may be stimulated by autocrine GF to increase production of PTHrP, leading to enhanced bone resorption. Tumor cells metastatic to bone may also secrete PTHrP, which can resorb bone and release additional GF which in turn act in a paracrine manner to further enhance PTHrP production.
Accordingly, compounds with activity to, for example, modulate PTH levels and/or calcium levels in vivo are desired.